The subject matter disclosed herein relates generally to imaging systems, and more particularly to correcting focal spot motion induced errors in detectors of the imaging systems.
Some known imaging systems, such as Computed Tomography (CT) imaging systems, include a source and a detector array as part of a gantry. The source and the detector rotate with the gantry within an imaging plane and around an object to be imaged such that the angle at which the beam intersects the object constantly changes. A scan of the object includes acquiring a set of views at different gantry angles, or view angles, during one revolution of the source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. The reconstruction process then converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units”, which are used to control the brightness of a corresponding pixel on a display.
In these imaging systems, the focal spot is the region from which the radiation projects. In some systems, the radiation produced diverges from the focal spot in a conical pattern. In order to produce an image from an axial scan with acceptable resolution, such as to provide clinically relevant image details, it is desirable for the focal spot to be properly aligned in the x-axis.
In particular, during operation, these imaging systems heat up due to different factors. The heat causes a thermal expansion of the some of the radiation source structures. The thermal expansion causes small mechanical displacements of source structures and a corresponding shift in the focal spot position. To correct for this shift, at least one known imaging system aligns the source at a single temperature, for example, an ambient temperature. However, as a result of the thermal drift, the single temperature alignment does not accurately reflect the position of the source focal spot during a scan of an object. The thermal drift of the focal spot then can cause aliasing and reduced image quality.
Focal spot drift may also result from the rotation of the gantry relative to the object being scanned, caused by the methods used for imaging system calibration, such as air calibration, misalignment of mechanical parts, and oscillation of the focal spot due to mechanical vibrations, among others. Thus, because perfect mechanical alignment of the focal spot is difficult or impossible to achieve in a commercial production setting and difficult to maintain in a clinical setting, systems and methods to mitigate focal spot motion/drift are used.